1. Field of the Invention
The present invention relates generally to the field of electrical test methods and equipment and more particularly to high-speed electrical testing of capacitors.
2. Description of Related Art
Rapid testing of capacitor leakage is important in the semiconductor industry. In the semiconductor industry, many replicate components, or die, are created on a single semiconductor wafer. Each of the individual die are electrically tested, commonly with a method called “probing.” During the probing process, a grid array of fine tungsten wires is touched down on the metallized bonding pads of each die. The tungsten wires are in turn connected to test equipment that is used to evaluate the electrical quality of each die. More specifically, the fine tungsten wires, or other contact media, known in the art as “probe card pins” or “probe pins,” are arranged on conventional printed circuit boards or test cards known in the art as “probe cards” or “probe array cards.” Probe cards are in turn connected to electrical test equipment known in the art as “probers” or “prober machines.”
Precision is important when testing electrical components but obtaining test results in a timely fashion is often equally important. This is especially true when many components are to be tested. An ideal capacitor that is charged to a steady state condition and disconnected from other components would hold its charge forever. However, certain intrinsic properties of real capacitors cause discharge over time. As depicted in FIG. 1, after a capacitor 10 is charged to voltage VC 16, extrinsic components 12 such as inadvertent connections or solder flux can cause or increase a leakage current 14 that results in discharge of capacitor 10. By measuring the leakage properties of capacitor 10, it is possible to determine if capacitor 10 meets its specifications, is installed properly, and whether connected or surrounding circuits are behaving as expected.
Referring now to FIG. 2, conventional methods of leakage testing test one capacitor 10 at a time. A voltage source initially provides voltage VC 16 to charge and soak capacitor 10. Once capacitor 10 is fully charged and soaked, a voltage source 20 provides a current to sense resistor 24 and the resultant voltage drop across sense resistor 24 is recorded as a measurement 220 (provided by voltage detector 22) of the amount of current required to counteract leakage current 14 such that capacitor 10 holds its charge. Various problems arising from this method to leakage measurement make the method unattractive for measuring complex systems that may contain hundreds or even thousands of large capacitors. One problem lies with the sense resistor 24. To achieve adequate resolution for very small currents, the conventional approach requires utilizing a large sense resistor 24. For example, a capacitor leakage current 14 of 5 nA would cause only a 500 μV across a 100 KΩ sense resistor 14 and, in the example, a test capacitor 10 having a capacitance of 100 μF would have a 10 second time constant. Such a circuit time constant would require approximately 150 seconds to charge, soak, and settle a capacitor 10.
Another problem with the method of testing is the limitation that only one capacitor can be tested at a time. For a circuit containing over 100 capacitors, it may take over four hours to obtain accurate leakage currents for the entire circuit. A conventional probe card may contain hundreds of capacitors that require repeated testing throughout the probe card's development and useful life. A manufacturer loses revenue for every minute that the probing process is inoperable, due to a probe card malfunction for example. What is needed in the art is a high speed method for accurately testing a multitude of capacitors.